1. Field of the Invention
This invention relates to a light emitting element (herein also referred to as LED element) and a method of making the same and, particularly, to a light emitting element that allows an enhancement in optical external radiation efficiency while preventing emitted light from being held inside a semiconductor layer with a high optical absorption coefficient, and a method of making the same.
2. Description of the Related Art
Conventionally, a method of making a light emitting element is known in which a semiconductor crystal of group III nitride-based compound semiconductor is grown on a substrate such as sapphire. There is, however, a problem in such a light emitting element that a light emitted in a light-emitting layer thereof is held inside a layer with a high optical absorption coefficient, and part of emitted light is absorbed in the layer, whereby the external radiation efficiency of emitted light lowers.
In order to solve the problem, a light emitting element is suggested in which its sapphire substrate is provided with an uneven surface, and a group III nitride-based compound semiconductor layer is grown thereon (for example, see Paragraph [0011] and FIG. 1 of Japanese Patent Application Laid-Open No. 2003-197961).
In the light emitting element of JP-A No 2003-197961, even when light enters into an interface between the sapphire substrate and the group III nitride-based compound semiconductor layer at a large incident angle (i.e., light that has a small angle defined between the interface and the incidence direction), the light can be externally discharged through a stepped surface (side face) of the uneven buffer layer, so that the light extraction efficiency can be improved.
However, the light emitting element of JP-A No, 2003-197961 involves such problem that, though the sapphire substrate is provided with the uneven surface to extract light (herein called intra-layer confined light) confined inside a layer rather than light directly discharged outside from the group III nitride-based compound semiconductor layer, the light extraction efficiency is not sufficient. Namely, since the light extraction characteristic depends upon the refractive-index difference between the group III nitride-based compound semiconductor layer and the sapphire substrate, the light extraction efficiency is limited
Moreover, since the group III nitride-based compound semiconductor layer has a high optical absorption coefficient, the intra-layer confined light must be attenuated so much therein and therefore the optical radiation efficiency cannot be enhanced.